Grow/bitmap: support adding bitmap via sysfs.

Grow/bitmap: support adding bitmap via sysfs.

Adding a bitmap via ioctl can only add it at a fixed location.
That location is not suitable for 4K-block devices.

So allow setting the bitmap location via sysfs if kernel supports it
and aim to always use 4K alignments.

Signed-off-by: NeilBrown <neilb@suse.de>